I am curious about this cool complex flying machine. I am planning to have one after my 450 size heli.

Does this unit (IRIS+) has a fail-safe RTL function like loss of signal from the tx / maybe weak tx signal?This may help prevent our units flying away to nowhere accidentally due to environmental factors.

Yes the IRIS+ supports this and is setup like this by default so that it will return back to you if it looses contact with your radio. It also has a low battery failsafe.

You can read more about the throttle failsafe (out of range failsafe) hereAnd for more information about the low battery failsafe you can find out more information here

The arducopter platform also supports other failsafe modes such as loss of GPS signal, loss of telemetry link, and also when the sensor fusion algorithm acts wierd. In each case you can configure what your IRIS+ will do, you can get it to just land where it is, or return back to you, or just hover in place.

Lastly if you are just starting out you can also use the geofence feature, so that your IRIS+ will only be able to fly a certain distance away from you. If you fly outside of the circle your IRIS autopilot will go to automatic control and return home to you.